Chemistry
of Dyeing Process
Dyeing
refers to the chemical process that entails the application of color to yarn,
fabric or fiber stock. Penetration of molecular colorant into the yarn or
fibers may or may not occur. It is possible to use dyes on manmade or animal
fibers affinity is present (Waring, 2013).
Some
examples of textile dyes are acid dyes and substantive or direct dyes. Acid
dyes are mostly used in wool dyeing while substantive dyes exhibit a strong
affinity for cellulose fibers. Additional salt or other chemical substances
should be used with mordant dyes to for affinity of a dye material. On the
other hand, sulfur dyes, used mostly for cellulose dying, lacks color
brilliance hence inexpensive. Insoluble pigments referred to as Azoic dyes form
within the fiber initially with a coupling soluble compound and later a
diazotized base. This entire process is referred to as padding. Sodium alkaline
hydrosulfite converts vat dyes (insoluble in water) into colorless soluble
compounds. Then, cellulose absorbs the resultant colorless compounds to be
oxidized in insoluble pigment formation.
Varied
dye colors result from distinct dye molecules. A particular dye molecule shape
absorbs light in a specific way. Molecules also make up clothing fabric.
Cotton, for instance, consists of long cellulose molecules strands in a twisted
pattern. There is no change when cotton and dye molecules are mixed together
unless atoms surface for reaction. A release of H and CL atom from cellulose
and dye molecule respectively prompts reaction and bonding. To release these
atoms, sodium carbonate (Na2CO3) should be added to the mixture of a dye agent
and cotton fiber. Sodium carbonate is a basic element that raises the pH level
for the oxidation to take place. Therefore, a high pH catalyzes the reaction
between the dye and cellulose.
Adding washing soda to the mixture of dye and
cotton seals the fate of the reaction. Therefore, adding sodium carbonate prior
to or after the reaction will yield a same outcome. During such period,
chlorine and hydrogen surface from dye and cellulose molecules respectively. In
optimal conditions, the dye is attached to the cellulose resulting in a unalterable
bond. Rinsing the cloth eliminates excess dyes.
Dyeing
with or without Mordant
A mordant forms a coordination
complex with dye. It then attaches to the tissue or fabric. A mordant is a
substance utilized in setting dyes on tissues or fabric. Its uses include the
intensification of stains in tissue or cell preparation and fabrics dyeing.
Early dyers were convinced that a mordant held a fiber onto a dye bite so that
it can hold firm when washing. It is composed of a polyvalent metal. Acidic or
alkaline colloidal ion results from dye coordination complex (Zolliger &
Iqbal 2011).
Common forms of dye mordant include
chrome alum, urine, tannic acid, and sodium chlorine. Others are chromium,
aluminum salts and iodine. Tin, potassium, and iron also have salt elements
usable as a mordant. While iodine is often viewed in Gram stains as a mordant,
it is a trapping agent. There are three methods of mordanting. The first method
is onchrome and involves treatment of substrates with a mordant. The second
method (metachrome) entails an addition of a dye bath. Lastly, the mordant is
used to treat the dye material. The nature of the mordant used not only affect
the dye fastness property, but also the shade outcome after the dyeing process.
Dye color properties are affected during the meta-mordanting stage because of
the saline traits of the mordant elements. The final dye color depends on the
mordant type and its reactive characteristics. In fact, each mordant reacts
differently with varied dye components. For instance, Dutch scarlet when used
with cochineal and tin mordant results in an orange color. If a mordant is not
applied, the dye will retain its original color.
Effect
of Acidity Variation of the Dyebath
Acid
dyes have sulphonic acids and other functional groups of anions. Mostly, they
attract amino groups that are positively charged on polyamide fibers like nylon
silk and wool. Many dye color change as the level of pH alters. Dye colorant
molecular structure determines the type of color. Therefore, dyes that belong
to anthocyanins class (consisting of red-blue-purple flavonoid are affected by
the changes in ph levels. A classic example is a litmus paper that is made up
of lichen dye. Hydrogen protons composed in such dyes increase or decrease
according to the acidity level hence resulting in color variations (Riegel
& Kent, 2013). Examples of natural dyes affected by pH include extracts
from cabbage, poppies, and blueberries.
References
Riegel,
E. R. ,
& Kent , J. A. Kent New York
Waring,
D. R., & Hallas, G. (2013). The
Chemistry and Application of dyes. New
York
Zollinger,
H., & Iqbal, A. (2011). Color
chemistry. Weinheim: Wiley-VCH.
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